The invention specifically pertains to the control of steering air in electrostatic rotary atomizers as they are conventionally utilized for the series coating of workpieces, e.g., car bodies. However, the invention can also be utilized in other types of atomizers. The invention is suitable for arbitrary coating materials, including coating liquids and coating powders.
In conventional rotary atomizers (DE 4306800), the steering air is directed from the atomizer onto the conical outer surface of the bell, wherein this steering air not only provides the coating particles that are radially expelled at the edge of the bell dish with an additional impetus in the direction of the workpiece, but it also serves to form the spray jet, and in part, to assist in the atomizing process. The steering air emerges from a circle of holes in the end face of a steering air ring arranged on the front end of the atomizer housing. The number, diameter, shape and direction of the holes may be chosen differently in order to optimize the air speed, the air quantity and the width of the spray jet. The respective steering air quantity that defines the desired spraying width is predetermined in the form of a parameter of the coating process, and is controlled in a closed control loop.
Arrangements of outlet openings in the form of annular slits can be provided for the steering air, instead of holes. In the rotary atomizer known from EP 0092043, an outer annular slit is provided in addition to a radially inner annular slit, wherein this outer annular slit is supplied with compressed air by the same source as the inner annular slit. The width of one or both annular slits is adjustable. The outer air curtain produced by the additional annular slit has the function of compensating for the marginal turbulence produced by the coating substance cloud in interaction with the inner air current, and of returning escaped coating particles back into the cloud.
In other rotary atomizers, radially outer auxiliary outlet openings for air are provided in addition to the radially inner air openings, wherein the air emerging from the auxiliary outlet openings is intended to prevent the coating particles from returning into the atomizer.
One general problem in the utilization of atomizers is that atomizing cones of different widths are required for coating different workpiece regions. Known high-speed rotary atomizer systems, for example, for coating car bodies, are preferably designed in such a way that, when coating large areas, bell dishes with a larger diameter are used and the width of the spray jet (defined as SB 50%, i.e., as the width at 50% of the maximum layer thickness of the individual profile) is adjusted to a value of approximately 300–550 mm. Smaller bell dishes are used for detail coating and interior coating processes, as well as for coating attachments and other small components such as mirrors, decorative strips and shock absorbers, where the width of the spray jet is usually adjusted to a value between 180–300 mm. When using smaller or narrower spraying patterns, the application efficiency, which is defined as the ratio between the coating material that is sprayed and the coating material that is precipitated, is higher than when using wider spraying patterns. This makes it possible to significantly reduce the costs, as well as the consumption of coating material.
EP 1114677 discloses an atomizer with exchangeable bells that differ from one another with respect to their diameter, spraying direction, and steering air quantity. The appropriate bell is chosen as a function of the shape of the object to be coated and the color used, etc. For example, a bell with a large diameter is used for exterior surfaces, and a bell with a smaller diameter is used for interior surfaces of car bodies.
If the coating process cannot be interrupted in order to replace the spraying head, high efficiency and a uniform coating layer can be achieved only by comprehensive coating of a workpiece with wide and narrow spray jet adjustments. Since it is not possible to adjust the spray jet to a sufficiently small size, it is necessary to make compromises with respect to the efficiency, the consumption of coating material, and the color shade, between the bell dish size with the corresponding steering air supply, and the jet width. Although superior constriction of the spray jet can be achieved by reducing the rotational speed, this results in a reduced atomizing fineness and deterioration of the coating quality. Until now, it has not been possible to optimally adjust the spray jet to both of the above-mentioned widths with the steering air of a given atomizer. This resulted in significant disadvantages in practical applications, for example, insufficient or impossible interior or detail coating processes, increased overspray (the portion of coating material sprayed past the object), low application efficiency, increased consumption of coating material, and insufficient coating quality.